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886

Appendix II.

of pipes through which the steam passes, and placed as near the
boiler as possible. Leaving the boiler at A, the steam enters the
two lower coils at B, then goes to the f after-evaporator' c, and the
uppermost coil D, from which it makes its way coil by coil to
leave at E for the engine. In this way wet steam circulates where
the heat is greatest, while the main and final superheat are more

effective by placing steam and gas currents in contrary directions.
The damper G adjusts tbe superheat, whicb is a maximum when G
is closed, and nothing wben open. This important attachment
thus meets changes in steam demand or stoking variation.

Compounding was explained at pp. 621 and 623. It has very
satisfactorily decreased initial condensation by diminishing -the
range of temperature in each cylinder, but is undoubtedly expen-
sive regarding first cost, a fact which presses less heavily on large
powers than on small ones. The gain is due to placing re-
evaporation at earlier positions in the total expansion.

Entropy and Temperature-entropy Diagrams.óLet
the meclianical energy given to an overshot water wheel be
w (H\~ H'2), where H'0 is the zero of head and energy, and w the
gmvity weight of the water used. Applying the analogy to heat
engines, we there have the heat energy given as ^ fa-^), where
r0 is zero of temperature and of energy, and <t> is what Zeuner calls